Capacitive sensing is now commonly used in high technology domains. In automotive vehicles for example, capacitive sensing circuits are often installed in occupant detection systems in order to control the triggering of security appliances such as airbags or seat belt pretensioners. It is now common to use capacitive sensing circuits called guard-sense capacitive circuits operated in loading mode.
The term “loading mode” is used to describe a concept of electric field sensing used for making non-contact three dimensional position measurements, and more particularly for detecting a presence and/or a position of a human body in relation with an electrode. The detection is deduced from the measurement of the current pulled from an electrode or transmitter plate through the human body, to the ground. This concept is to be distinguished from the “shunt mode”, and the “transmit mode”, which are the two other common capacitive sensing modes. For further explanations about the different capacitive sensing operating mode, refer to the technical paper entitled “Electric Field Sensing for Graphical Interfaces” by J. R. Smith, published in Computer Graphics I/O Devices, Issue May/June 1998, pages 58-60.
A guard-sense capacitive sensor as it is commonly known in the art comprises a sense electrode connected to a sense node, and a guard electrode connected to a guard node. The sensor further comprises a periodic voltage source connected to the guard node for providing, in operation, a guard voltage of a predetermined amplitude to the guard node.
The sensor further comprises a control and evaluation circuit connected to the sense node and the guard node. The control and evaluation circuit is configured to, in operation, keep the sense electrode at the same potential as the guard electrode by injecting a current to the sense node which corresponds to the current which is drawn from the sense electrode via the unknown sense impedance to be determined. The space between the two electrodes is thus free of an electric field, and the sense electrode becomes insensitive to a body located anywhere in the direction of the guard electrode.
It is understood that in the context of the present application we use a common theoretical approximation to assert that the sense and the guard electrode are kept at the same potential and that the space between said two electrodes is free of an electric field. This approximation will also be used in the following description.
The control and evaluation circuit is configured to determine the current it is injecting to the sense node and to issue an output signal which is indicative of the current injected into the sense node and thus of the sense impedance to be determined. In order to perform these tasks, the control and evaluation circuit commonly comprises a transimpedance amplifier and an analog to digital converter. The negative and positive inputs of the transimpedance amplifier are respectively connected to the sense node and the guard node. The output signal of the transimpedance amplifier is proportional to the current injected by the amplifier into its negative input in order to keep the difference of potentials between its inputs equal to zero. The analog to digital converter is usually connected to the output of the amplifier.
The guard-sense capacitive sensing circuit has the advantage of being directional and capable of detecting a body without being perturbed by non-dielectric surrounding objects. Nevertheless, the measuring circuit commonly uses non-ideal components inducing a non-negligible error in the determination of the sense impedance that can be critical at low impedances.
Generally, the error in the determination of the sense impedance is associated with various unknown impedances, of which the main source is the sense-to-guard impedance. It is understood that the guard-sense impedance is the impedance between the guard node and the sense node. The guard-sense impedance is also called hereafter parasitic impedance.
There are already solutions known in the art to measure time variant non-ideal device impedance by suppressing the influence of the parasitic impedance in capacitive measurement circuits. For example patent document U.S. Pat. No. 4,481,464, describes a solution to improve the impedance measurement in a non-ideal capacitive measurement circuit. The circuit comprises an input portion including a pulse generator, a sine wave generator and performs a demodulation in order to separate the interference component of the measured signal corresponding to the parasitic impedance.
Such a solution however only applies to capacitive sensing systems operating in coupling mode and is not applicable to loading mode measurement to which the present invention relates.